I. Field of the Invention
This invention pertains to an electrical resistance welding apparatus for making terminal welds in lead storage batteries.
II. Description of the Prior Art
Lead storage batteries are often made with straps connecting the individual plates in a stack together, the strap having a vertical portion called a lug. Terminals are provided in the battery for connection of the battery to external electrical cables, the terminals in some batteries being positioned in the battery side walls. A portion of the surface of the lug overlaps a portion of the inside surface of the terminal and the two must be connected together to form a good electrical connection. In one approach, the overlapping surfaces of the lug and terminal are pressed together by electrodes and an electrically welded connection is formed. One problem with this approach is that the overlapping area of contact is large creating a low electrical resistance resulting in insufficient heat at the interface thereby causing a cold weld.
Alternatively, it is known to provide the lug portion with a projection which is inserted through a hole in a battery wall and into a hole or cavity in the terminal. The complete connection between the terminal and the lug and a good seal between the terminal, battery wall and lug is obtained by exerting pressure on either side of the connection, that is, by applying pressure against the battery lug within the battery and against the terminal outside the battery so as to squeeze them together and seal the connection against the battery wall. If desired, the step of fusing the connection together can be added by applying heat to the lug and the terminal. See U.S. Pat. Nos. 3,457,118 and 3,303,056. Other methods for producing battery terminals which do not require the use of an electrical resistance welder are described in U.S. Pat. Nos. 3,849,203 and 3,874,933.
Various electrical resistance welding apparatuses and methods for making intercell connections within batteries are disclosed in U.S. Pat. Nos. 3,869,316; 3,798,071; 3,793,086; 3,723,699; 3,687,734; 3,598,948; 3,388,005; and 3,313,658. In particular, the Badger U.S. Pat. No. 3,793,086 discloses a method of making an intercell connection by placing flat surface connector lugs on each side of a battery partition wall, adjacent an aperture in the wall. Electrodes under the application of high shear force extrude the lugs into the aperture until they meet. An electrical welding current is then applied until the welded connector lugs fill the aperture with metal. The welding current then ceases and the welded connector lugs are allowed to cool and the electrodes are removed from connection in the battery casing, leaving behind a liquid-tight battery connection.
The Katz U.S. Pat. No. 2,583,575 relates to a machine for spot welding two sheets of steel through an aluminum core, the aluminum core comprising a sheet of aluminum with a hole in it. The patent discloses that the electrodes of the welding machine push in from either side on the sheets in the area enveloped in the hole and thus force the sheets of steel toward one another until they touch. Then current is applied to the electrodes and fusion occurs.
As the prior art above shows, even though electrical welding methods are known to be used for making intercell connections within a battery, the prior art does not show the use of using electrical resistance welders to construct storage battery terminals without the use of special projections, etc. on the lug or terminal or both. See the battery terminal patents enumerated above.
Today, battery terminals are cast into the walls of batteries with a portion of the casting being located on the inside surface of the battery casing adjacent to an overlapping portion of a battery lug. See U.S. Pat. No. 3,874,933. In some instances, the casting provides a boss on this inside surface. However, the area of contact between the overlapping lug surface and the terminal surface or terminal boss surface is relatively large and results in a low resistance contact thereby lowering the amount of heat generated at the interface when current is applied to the electrodes. Because a great deal of heat is required in order to melt the lead and fuse the parts together to form the terminal connection, this low electrical resistance is not desirable. Also, the requirement of projections, etc., on the terminal and/or lug which are fixed in position once they are placed in the battery can cause serious alignment problems.